ENH: stats: Add Mood's median test.

This test is described in many statistics text books.  Google for it and you'll find many descriptions and tutorials about it.  It's about time we had it in scipy.  Since we already have the contingency table code, the implementation is pretty simple.
vectorize exception?
`if (colsum == 0).any():  "you got zeros in columns np.nonzero(colsum == 0)[0]"`
(instead of shortcircuiting and raise on first)